Two standards are known for the color space of digital video signals. These are ITU-R BT.601 and ITU-R BT.709, which are specified by the ITU-R (International Telecommunication Unit, Radiocommunication Bureau). For example, an RGB signal that is captured by a camera is converted to a YCbCr signal, which is a signal of the luminance and the chroma, and recorded and transmitted, and in the display is reconverted to an RGB signal and the color and tone (grayscale) are reproduced. Here, the RGB signal can be expressed in a three-dimensional color space that is specified by the non-linear primary colors R, Gy and B (0 to 1) that are obtained by processing with a gamma value for gamma correction of approximately 1/2.2.
On the other hand, compared to this non-linear RGB signal, the YCbCr signal is specified such that the Y signal, which is the luminance, of the YCbCr signal ranges from 0 to 1. Next, the two chroma signals Cr and Cb are defined in terms of R-Y and B-Y, respectively, and the range that these chroma signals can take is specified to a range with a minimum value and a maximum value of (−0.5 to 0.5) so as to just contain a RGB color three-dimensional object. The signal values that are specified by values from 0 to 1 are encoded to an 8-bit digital value, but not all values (0 to 255) in 8 bits can be taken for the Y signal, the Cr signal, and the Cb signal. This is because a margin must be secured in order to reliably perform various processing using the video signal (moving picture). In practice, the offset value and the gain are set so that the dynamic range of the Y signal is a value in the range of 16 to 235, and so that the Cr signal and the Cb signal have a colorless gray value of 128 and the dynamic range is a value from 16 to 240.
Consequently, the colors in 8-bit conventional digital video signals are in the range of 16 to 235 for the Y signal and 16 to 240 for the Cr signal and the Cb signal, and thus video device hardware and software for handling video executed processing on all signals in this range.
To provide a more detailed explanation, the actual Y signal, Cr signal, and Cb signal are limited to the above ranges, and furthermore only colors that are within the RGB (0 to 1) space corresponding to the signal are defined. If the signals are to be converted to RGB, then those YCbCr signals that correspond to regions where the converted RGB values are negative or greater than 1 would in practice not be defined, even if they could exist.
The color gamut with a Y signal value of 16 to 235 and Cr signal and Cb signal values of 16 to 240, in which the RGB values that correspond to the YCbCr values are within a space from 0 to 1, is referred to as the BT709-RGB color gamut, and that color space is referred to as the BT709-RGB color space. Reference numeral 201 in FIG. 2 indicates a BT709-RGB color gamut.
In recent years, devices known as extended color gamut displays that can express colors beyond the conventional RGB space, particularly liquid crystal displays for reproducing video, have entered the market. For example, liquid crystal TVs that use an LED backlight with three or six primary colors have been released, and these are furnished with a display that can express the colors of a color gamut that exceeds the conventional color gamut, some even being furnished with a display that can express 95% or more of the surface colors in the natural environment. These devices are intended for commercial sale, and thus the video source that is input is a conventional video standard based on ITU-R BT.601 and ITU-R BT.709. This leads to the problem that the features of the extended color gamut display cannot be taken advantage of because the only color range that is available for expression is the conventional BT709-RGB space.
Accordingly, a color space encoding method that can maintain continuity with the conventional standard while allowing for expression of an extended color gamut by also using regions outside of the BT709-RGB color space that conventionally have not been used, has been proposed as xvYCC and steps toward standardization are underway. The xvYCC color space maintains compatibility with conventional ITU-R BT.601 and ITU-R BT.709 while broadening the color space. Consequently, it is possible to record, for example, digital video signals within the xvYCC color space without changing encoders or decoders for the conventional video format or MPEG and the like, or media standard of DVDs and the like. Further, the xvYCC color space is an extended color gamut space and thus it is possible to record video capturing an object that has a wide color gamut, such as the negatives of color photographs, and video in which color adjustment or the like has been performed using a wide color gamut, as is without loss.
Conventionally, various types of color correction processing have been executed in order to make captured color video appear prettier, but the RGB range is quickly exceeded when such processing is performed, making it impossible to feel the effect of the wide-band color reproduction. Using the xvYCC color space, which has a wide color space, allows for the expression of colors that heretofore could not be reproduced, and thus it is envisioned that the color adjustment (color editing), etc., of color images will be exploited more than ever before, allowing the effect of wide color reproduction to be experienced on wide-color-gamut TVs and impressing many people.
However, if color editing is performed assuming an xvYCC color space in video that has been captured or color video that is created on a computer, and then that color-adjusted signal is transmitted, then the current status of that transmitted signal will be unclear, and as a result it is conceivable that color-adjusted video will be subjected to additional color adjustment.
To solve this problem, Patent Citation 1, for example, discloses adding a header block to a plurality of images to indicate how the color and tone of the images are adjusted. This method requires two tags among the varied information for describing content, one for indicating which of the various standard color spaces is used, and one for specifying the color adjustment and the color editing that have been executed within that color space. By adding a flag that specifies the color space xvYCC and indicates that color adjustment has taken place, and another flag that indicates the manner of that adjustment, to digital video as well, it is possible for the device that recognizes this to avoid performing color adjustment a second time and to perform processing to reverse the color adjustment that has already been performed, and thus the problem discussed above can be avoided.    Patent Citation 1: Japanese Laid-Open Patent Publication No. 2003-153025 (pgs. 4 through 6, FIG. 1)